Grohtherm 3000 (19665) - Boiler not recognising demand for hot water

[Fleetman]

When attempting to use the shower our combi-boiler does not fire up in response to the demand for hot water. However, if using the Exafill to run water into the bath then the boiler DOES fire up - as it does also with other hot taps in the house.
British Gas service engineer has checked the boiler and says that the fault is with the shower not the boiler.
Anyone familiar with this model of shower who can assist with a possible diagnosis?

 

[steadyon]

The shower has non-return valves inside, under the two brass covers either side of and just above the temperature control. Worth taking these out (requires 8mm allen key) and making sure they are free. If stuck, shower may draw insufficient hot to fire boiler.

Note. Water off first!

 

[Fleetman]

The shower has non-return valves inside, under the two brass covers either side of and just above the temperature control. Worth taking these out (requires 8mm allen key) and making sure they are free. If stuck, shower may draw insufficient hot to fire boiler.

Note. Water off first!

Thanks for the advice. Much appreciated!